In modern semiconductor devices, the ever increasing device density and decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques of such devices. Conventionally, a flip-chip attachment method has been used in the packaging of IC chips. In the flip-chip attachment method, instead of attaching an IC die to a lead frame in a package, an array of solder balls is formed on the surface of the die. The formation of the solder balls is normally carried out by through-mask evaporation, solder paste screening, electroplating through photoresist, or injection molding of solder.
U.S. Pat. No. 5,244,143, which is commonly owned by International Business Machines Corporation, discloses the injection molded solder (IMS) technique and is hereby incorporated by reference in its entirety. One of the advantages of the IMS over other solder bumping techniques is that there is very little volume change between the molten solder and the resulting solder bump. The IMS technique utilizes a solder head that fills molds comprised of boro-silicate glass, molybdenum, silicon, polyimide, and the like that are wide enough to cover most single chip modules. A narrow wiper can be optionally provided behind the solder slit passes the filled holes of the mold to remove excess solder.
The IMS method for solder bonding is then carried out by applying a molten solder to a substrate in a transfer process. When smaller substrates, i.e., chip scale or single chip modules are encountered, the transfer step is readily accomplished since the solder-filled mold and substrate are relatively small in area and thus can be easily aligned and joined in a number of configurations. For instance, the process of split-optic alignment is frequently used in joining chips to substrates. The same process may also be used to join a chip-scale IMS mold to a substrate (chip) which will be bumped.
A problem common to the solder ball forming techniques discussed above and other techniques not discussed such as molten solder screening is with that a mold or die needs to be used. Current molds are limited to a rectangular form and have cavities arranged in a pattern specific to a pattern of a substrate design. In other words, current molds can only be used for a particular substrate design. Every new or change in design requires a new build of a mask or mold. This is true for existing technologies of plating and evaporation, as well as with IMS. In most cases, it is also preferable to produce multiple copies of masks or molds for throughput or redundancy. The costs for these new masks and molds will vary significantly, but are costly. This also drives delivery time, which can gate the delivery of final parts. This is especially costly in the event of a redesign, which can add several weeks onto the delivery schedule.
One problem with several of the mold materials, including borofloat, kapton, polyimide on glass, is that an existing infrastructure for building the molds does not exist. Unlike glass masks used in plating of solder, or metal masks used in evaporation of solder, both of which are readily available internationally, mold fabrication does not exist in mass production. While infrastructure does exist for some mold fabrication, such as molybdenum, this suffers from other disadvantages.
Another problem with using molds for depositing solder onto a substrate is that current molds are rectangular. Therefore, the mold and the solder head are moved linearly with respect to each other such that the cavities move perpendicular to a slit in the solder head thereby filling the cavities as they pass.
Therefore a need exists to overcome the problems with the prior art as discussed above.